The present invention relates generally to a zoom optical system and an imaging apparatus using the same, and more particularly to a zoom optical system that, albeit having high zoom capabilities, has its total length curtailed in consideration of size reductions upon received at a lens mount and an imaging apparatus incorporating the same.
In recent years, imaging apparatus using an electronic imaging device, like digital still cameras, have gained wide popularity. With the development and widespread of digital still cameras, there have been growing demands for image qualities, functions and sizes, and slimmed-down yet high-zoom-ratio zoom optical systems in particular are now in great demand.
Among optical systems having a zoom ratio of about 3, zoom optical systems comprising a first lens group of negative refracting power and a second and a third lens group of positive refracting power have been well known in the art. With an optical system having a negative group at the front, however, there is much difficulty in making sure brightness when it has a high zoom ratio especially on the telephoto side. Accordingly, an arrangement comprising a first lens group of positive power, a second lens group of negative power, a third lens group of positive power and a fourth lens group of positive power is often used for an optical system having a high zoom ratio.
Even with an optical system having a positive group at the front, however, its optical length is likely to get long as the zoom ratio grows high.
Size reductions of an optical system, too, are an important challenge for size reductions of imaging apparatus; it is important to curtail just only the total length of the optical system but also the total length of each lens group.
Some proposals have been put forward to meet such demands.
Patent Publication 1 discloses an optical system having a zoom ratio of about 5, showing in one example that the number of lenses is 9 with a reduction in the thickness of each lens group. However, the telephoto ratio at the telephoto end is 1.7 to 1.9: the maximum total length is still somewhat long.
On the other hand, Patent Publication 2 discloses an optical system having a zoom ratio of about 6.6, showing in several examples that the number of lenses is 9, and 10 with a reduction in the thickness of each lens group; further improvements in the zoom ratio and size are made while the telephoto ratio is kept at as low as about 1.5 with high zoom ratios.
With those prior arts, however, there is much to be desired in the total length because the telephoto ratio exceeds 1.5. For size reductions upon received at a lens mount, it is required to make group-to-group spaces narrower while just only the total length but also the amount of movement of each lens group is reduced.
Patent Publication 1
JP(A)2004-94233
Patent Publication 2
JP(A)2005-181499